1. Field of the Invention
This invention relates to a process for the decontamination of oil-contaminated particulate solids, especially oil-contaminated particulate solids as removed from an oil well bore. In particular, it relates to the cleaning of oil-contaminated drill cuttings, and the separation and removal of oil therefrom.
2. Background and Prior Art
In rotary drilling, as practiced in oil and gas exploration, a well bore is formed which extends downwardly from the earth's surface to an oil or gas producing stratum. Formation of the well bore requires cutting into the earth with a rotating bit attached to the end of a drill string formed from joints of pipe sequentially attached as the well bore is extended downwardly. Various fluids are employed in well bore formations, as for drilling, completions and workover operations. Exemplary of these are (1) drilling fluids, or "muds", which are clay-based, and (2) brines which are clay-free, clear, heavy (dense) fluids. Drilling fluids, or muds, are generally used, and clear fluids or brines less often, during drilling operations. Brines are most often used for well completions and workovers. By definition, a workover is any operation in the well bore other than drilling. By this definition, completion operations comprise the first "workover".
In drilling the well bore, generally a clay-based drilling fluid, or mud, constituted of a mixture of weighting materials, clays, chemicals and water or oil, is pumped downwardly through the drill string as the well is drilled to exit through jets in the drill bit at the bottom of the hold, the mud ascending to the surface via an annular space between the exterior wall of the drill string and the wall of the hole, or well bore, which may be walled with well casing. At the surface, the mud is pumped to a shale shaker equipped with one or more layers of vibrating screens of one to three, or more mesh sizes for drill cuttings removal. The mud, after removal of the drill cuttings, is then returned to a mud pit where it is temporarily stored for reuse. The drilling fluid, or mud, or brine if it is used in drilling operations, serves several essential functions, the most important of which is to (1) control subsurface pressures and maintain stability, (2) cool and lubricate the drilling tool or bit, (3) suspend and transport to the surface all movable solids, notably drill cuttings, (4) provide rheologically-stable flow or circulation for the purposes characterized in (2) and (3), supra, and (5) provide a chemically stable or compatible chemical environment within the well bore. The chemically stable environment will minimize or eliminate corrosion of the drill string and casing as well as provide a compatible environment to the formations encountered in order to seal off permeable formations of oil, gas or water as the well is drilled through different subterranean formations and strata. In the event of a shutdown in the drilling operation, the purpose of the mud, or brine if used in drilling operations, is to hold the cuttings, sand and other solids particulates, or residual solids materials in suspension within the column of drilling fluid while efforts are being made to maintain or restore circulation of the drilling fluid.
In U.S. Pat. No. 3,737,037 which issued on June 5, 1973 to Lee Bone, III, there is described a process for treating drilling mud to remove substantially all of the suspended solids particles to provide a reusable mud. The drill cuttings, as is conventional, are removed via use of a shale shaker and the mud is then dumped into a mud pit. The smaller solids particles of size ranging down to about 20 microns in diameter are separated from the mud downstream of the shale shaker and mud pit by use of a series of centrifuges. A conventional decanting centrifuge is employed in series with a cyclone centrifuge, the decanting centrifuge being employed to remove the weighting solids, or barite particles from the mud, while the cyclone centrifuge is used to remove the smaller particles from the mud. Downstream of the series of centrifuges, the solids of size smaller than 20 microns re removed from the mud by the addition of flocculating agents to a tank of the mud, and the mud is then filtered to remove the flocculates. The mud, essentially free of solids, is then returned to the mud pit where it is available for reuse. There is no mention whatever, inter aba, within the metes and bounds of the bone patent of treating clear brine drilling fluids.
The drill cuttings when brought to the surface, and separated from the mud, are contaminated with oil. Residual mud solids recovered from the mud pit may likewise be highly oil-contaminated. In fact, at one time it was highly desirable to use oil based muds comprising various solids mixed with diesel oil, e.g. typically 1 to 15 percent by volume diesel fuel, for torque reduction or viscous drag reduction on the drill string to suppress or prevent sticking of the drill string. This very useful practice, once common, is not now often used because of the oil contamination and oil-slick problem. Oil contamination of these solids, particularly the drill cuttings, presents a pressing disposal problem for the solids cannot be simply dumped because of oil pollution problems. This means, of course, that diesel oil is rarely ever added to muds solely for lubrication purposes, despite the advantages. Instead, more expensive mineral oils are used in oil-based muds on a permit basis. Significant economies may also be achieved due to the desire, and need to recover the oil, but are particularly due to the need to dispose of a troublesome waste material. Thus, there is a pressing need for processes suitable for decontamination of the oil-contaminated solids so that they may be discarded or discharged without causing pollution of the environment. Also, there is a need for recovery of the oil separated from the solids and a need to clean up the water used in drilling a well.
It is, accordingly, the primary objective of this invention to fulfill these needs.
It is a specific object to provide a process for the cleaning of oil from oil-contaminated particulate solids, particularly oil-contaminated drill cuttings, and for the demulsification, separation, and recovery of the oil removed from the solids.